iuguy5 saidI don't think it's particularly fair to charge people based on their weight. First nobody chooses their genetic makeup, and some people are going to be heavier no matter how healthy they eat and how much they exercise. Your bag weight, on the other hand, is entirely in your control. But if you don't fit in one seat, that is a different story---I favor making obese people buy multiple seats if there is going to be someone next to them....or maybe charging more for their seat and discounting their neighbor a bit since they will be getting less of a seat than a person with a thin seatmate. But this shouldn't be punitive, just practical.
The statement I bolded above should be the guiding principal here. An airline is in the business of transporting pounds, not people, although it may be romantic to think otherwise. Transporting pounds from one place to another -- whether those pounds are in your belly, in your bag, or in your bicep -- costs money. Is it fair to expect airlines to disregard the most basic principle of their business?
What I described a few posts above could be called "capitalism," and what you described could be called "communism." They are both, ironically, systems meant to be just, and societies must decide which they prefer.
What people weigh is determined in part by factors beyond their control and in part by factors within their control. The same is true of what their luggage weighs. Someone who, through no fault of his own, suffers from sleep apnea and must carry around an iron lung would have far heavier luggage than someone who doesn't.
I don't see why fitting in one seat should be an exception to your general policy of "from each according to ability; to each according to need."
Of course, I weigh about 20 pounds and travel light, so I would fly for free if tickets were sold based upon weight.